Amosite asbestos was found to be 100 times more cytotoxic for human mesothelial cells and 10 times more cytotoxic for normal human bronchial epithelial cells than for normal human bronchial fibroblasts. Chrysotile was the most cytotoxic fiber tested; glass fibers were also very cytotoxic for mesothelial cells. Focal hyperplasia and epidermoid metaplasia were observed in explants of human bronchial tissue two weeks after a single exposure to amosite asbestos; both intracytoplasmic and intranuclear asbestos fibers were seen by X-ray microanalysis in these lesions. Two subculturings after amosite asbestos exposure of phenotypically altered mesothelial cells arose. The control cultures ceased growth during the fourth subculture, and the amosite asbestos-exposed cultures continued to multiply. The exposed cells had a near-normal model number of chromosomes through the sixth subculturing. At the ninth subculture, 80% of the metaphases had dicentric chromosomes and the model number had increased to 77.